Epidermoid cysts are small, reddish or white bumps that can appear anywhere on the skin. They may also (rarely) occur in bone. When present under the fingernail, they are called subungual epidermoid cysts. In this location they can be single but are more often multiple. Usually, only one digit is affected, especially the thumb. Epidermoid cysts are slow growing, typically painless, and almost always benign. Therefore, if a cyst grows rapidly, the diagnosis should be reassessed. They very rarely develop into skin cancer. Although epidermoid cysts usually do not require treatment, they may become inflamed, leading to swelling and pain. They also can rupture and become infected. There are no preventive measures. There are multiple synonymous terms used to identify the epidermoid cyst. Other names included epidermal inclusion cyst, wen, steatoma and sebaceous cyst. Some authors identify the sebaceous cyst as a separate and distinct lesion because its cells secret sebum into the cyst while the invaginated epidermis of the epidermoid cyst fills the lesion with layers of keratin. Clinically the epidermoid cyst is typically a firm (almost hard) white small mass which can have layers like an onion when cut. The sebaceous cyst is filled with a liquid or pasty sometimes foul-smelling material (sebum). In a practical sense probably, these lesions represent a continuum that all related to congenitally or traumatically invaginated epidermis.1,3,4
Pathophysiology
- Congenital epidermoid cysts may be due to abnormal ectodermal implantation during embryogenesis.
- The acquired variety occurs as a result of trauma or skin disorder (eg, acne).
- The cysts form when epidermal cells penetrate deep into the skin and multiply rather than follow the normal course and shed.
- The cyst wall comprises these cells, which secrete keratin into the cyst.
Related Anatomy
- Epidermoid cysts are most commonly located on the face, neck, and trunk but also see in the fingers, thumb and hand.
- Intraosseous epidermoid cysts occur most frequently in the distal phalanges and the skull.
- In extremely rare cases, epidermoid cysts can develop in the brain, spleen, testes, liver, kidney, and cecum. These cysts are usually found incidentally, while performing imaging studies for another reason.
Incidence and Related Conditions
- Risk factors for epidermoid cysts, which may be congenital or acquired, include history of acne, post-pubertal life stage, rare genetic disorders (eg, Gardner syndrome), injury/trauma (eg, surgery)
- The typical epidermoid cyst is usually not brought to the attention of a physician, so overall incidence is unknown.
- The term “sebaceous cyst” is often used interchangeably with “epidermoid cyst,” but the sebaceous cysts are a different entity and are less common. Sebaceous cysts arise specifically from oily sebaceous glands of the skin. Sebaceous cysts usually have an edge located in the most superficial layer of the skin. They are truly cystic and frequently contain a milky or pasty material. They can drain spontaneously and get secondarily infected.
Differential Diagnosis
- Boil
- Brodie abscess
- Glomus tumor
- Sebaceous cyst
